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is it true, oscilloscope must reach at least 4x observed freq?

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oxy:
Hi,

some people say, if I wanna observe 100MHz, I must use an oscilloscope that reaches at least 400MHz.

Is it true?

bdunham7:
The first step in understanding this question is to accurately explain what you mean by "100MHz".

coppice:

--- Quote from: oxy on September 12, 2022, 04:43:14 pm ---Hi,

some people say, if I wanna observe 100MHz, I must use an oscilloscope that reaches at least 400MHz.

Is it true?

--- End quote ---
Are you talking about bandwidth (100MHz) versus the sampling rate needed to achieve that in a digitising oscilloscope? By the Shannon/Nyquist/Whittaker/Kotelnikov sampling theorem you will need a sampling rate more than 2 times the bandwidth. So, by theory you need a sampling rate of 200Msps. In practice you need something a bit higher than that. Many scopes use something like 2.5x, while others use something considerably higher, like 4x or 5x.

Fungus:

--- Quote from: oxy on September 12, 2022, 04:43:14 pm ---some people say, if I wanna observe 100MHz, I must use an oscilloscope that reaches at least 400MHz.
Is it true?

--- End quote ---

No. The minimum needed would be 250Mhz to observe a 100Mhz sine wave.

That's why you see (eg.) 100Mhz, 4-channel 'scopes with 1GHz sample rate.

bdunham7:

--- Quote from: Fungus on September 12, 2022, 04:55:21 pm ---No. The minimum needed would be 250Mhz to observe a 100Mhz sine wave.

That's why you see (eg.) 100Mhz, 4-channel 'scopes with 1GHz sample rate.

--- End quote ---

MSa/s and MHz are two different things and the way you put it is very confusing at best.  The OP didn't actually specify a DSO and from the nature of his question, I don't think he's referring to sample rates.

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